Wavefront sensors are devices used to measure the shape of the wavefront of a light beam (see, for example, U.S. Pat. No. 4,141,652). In most cases, a wavefront sensor measures the departure of a wavefront from a reference wavefront or an ideal wavefront such as a plane wavefront. A wavefront sensor can be used for measuring both low order and high order aberrations of various optical imaging systems such as the human eye (see for example, J. Liang, et al. (1994) “Objective measurement of the wave aberrations of the human eye with the use of a Hartmann-Shack wave-front sensor,” J. Opt. Soc. Am. A 11, 1949-1957; T. Dave (2004) “Wavefront aberrometry Part 1: Current theories and concepts” Optometry Today, Nov. 19, 2004 page 41-45). Furthermore, a wavefront sensor can also be used in adaptive optics in which the distorted wavefront can be measured and compensated in real time, using, for example, an optical wavefront compensation device such as a deformable mirror. As a result of such compensation, a sharp image can be obtained (see for example, U.S. Pat. No. 5,777,719).
Currently, most of wavefront sensors designed for measuring the aberration from human eye are Shack-Hartmann type, in which the measured wavefront is simultaneously divided in a parallel format into many sub-wavefronts. The essential components of such a sensor include a light source or input optical beam, an array of tiny lenses (called the lenslet array), and a camera or some other means for recording the pattern and location (also called centroid) of the spot images formed by the lenslets array.
FIG. 1 shows an exemplary prior art Shack-Hartmann sensor used for eye aberration measurement. An SLD (superluminescent diode) 102 is generally used as the light source and the light is delivered through the eye's optics (including the cornea 104 and the crystal lens 106) to a relatively small area on the retina 108. The scattered light from the retina 108 travels through the eye's optical imaging system (including the cornea 104 and the crystal lens 106) and emerge from the pupil as an aberrated wavefront 110. In order to suppress interference from light reflected by the cornea 104 and other optical interfaces such as those of the crystal lens 106 other than the retina 108, the input relatively narrow light beam is usually polarized by a first polarizer 112 in a first direction. Given that light scattered by the retina is much more depolarized, the retina scattered light is usually measured in a second orthogonal polarization direction with a second orthogonal analyzer 114.
One can use a relay optics system, for example, 116, consisting of a set of lenses, to magnify or de-magnify or simply transfer the aberrated wavefront onto a lenslet array 118. If the lenslet array 118 is in a pupil conjugate plane (an image plane of the pupil), the wavefront at the lenslet plane will be identical to or will be a magnified or demagnified version of the wavefront shape at the eye's pupil. The lenslet array 118 then forms an array of spot images on the CCD camera 120. If the eye is a perfect optical system, the wavefront at the lenslet array plane would be perfectly flat (as shown by the dashed straight line 122) and a uniformly distributed array of image spots would be recorded by the CCD camera 120 located at the focal plane of the lenslet array.
On the other hand, if the eye is not perfect, the wavefront 124 at the lenslet array will no longer be perfectly flat and will have irregular curved shapes. Consequently, the spot images on the CCD camera 120 will depart from the location corresponding to the aberration-free case. Through data processing of the image spot position on the CCD camera 120, both low order and high order aberrations of the eye can be determined (see for example, J. Liang, et al. (1994) “Objective measurement of the wave aberrations of the human eye with the use of a Hartmann-Shack wave-front sensor,” J. Opt. Soc. Am. A 11, 1949-1957).
Although a wavefront sensor can measure both the low order and high order aberration of an optical imaging system, for a non-static imaging system such as the human eye, it has been shown that only low order aberrations corresponding to the sphero-cylindrical error measured from the central portion of the eye are relatively consistent (see for example, Ginis HS, et al. “Variability of wavefront aberration measurements in small pupil sizes using a clinical Shack-Hartmann aberrometer” BMC Ophthalmol. Feb. 11; 2004 4:1.).
In practice, for most eye aberration measurements and correction as well as for most fundus imaging optical systems, the optical aberrations that need to be measured and corrected are the sphero-cylindrical error (also called defocus and astigmatism). It is well known to those skilled in the art that these aberrations can be measured using a small number of sub-wavefronts around an annular ring of the input wavefront. In such a case, a large portion of the CCD detector arrays read out would be wasted. In order to save cost, a number of (typically 8 or 16) quad-detectors can be arranged around an annular ring of an aberrated wavefront to make the measurement of only these sub-wavefronts (see for example, U.S. Pat. No. 4,141,652, which, together with all other references cited, are herein incorporated in their entirety as references for this patent application).
However, with this arrangement, it is still necessary to use a multiple number of quad-detectors, which, although, are collectively less expensive than a large area CCD camera, but are still more costly than a single quad-detector. In addition, alignment a number of quad-detectors will also be much more difficult than that of a single quad-detector.